For example, materials for valve springs for automobile engines may include oil tempered carbon steel wires (SWO-V), oil tempered chromium-vanadium steel wires (SWOCV-V), and oil tempered chromium-silicon steel wires (SWOSC-V), which are specified in the Japanese Industrial Standards (JIS). The oil tempered chromium-silicon steel wires are conventionally widely used in view of the fatigue resistance and the sag resistance. In recent years, reduction in weight of the valve spring is strongly desired in order to improve fuel efficiency of automobiles, and there is a trend of increasing tensile strength in spring wire so as to increase design stress of the valve spring. In a case of a spring wire such as an oil tempered wire specified in the JIS, notch sensitivity to cracks and defects such as inclusions is greatly increased according to the increase in the strength of the spring wire. Therefore, such a spring wire is more likely to break during cold spring forming (coiling) and for brittle fracture to occur while in use. In a coil spring after it is coiled, tensile residual stress is generated in a direction in which external compressive force is applied during the coiling, and compressive residual stress is generated in a direction in which external tensile stress is applied during the coiling. Therefore, a spring wire with higher tensile strength tends to have these residual stresses at greater values. In addition, when a coil spring is compressively deformed, highest tensile stress is applied on a surface at an inner diameter side of the coil spring. Therefore, when a cold-formed coil spring is compressively deformed, high tensile stress is applied on the inner diameter side of the coil spring in addition to the tensile residual stress that is generated after it is coiled. Accordingly, the inner diameter side of the coil spring is likely to decrease of fatigue strength.
In order to correspond to this, high compressive residual stress may be provided on a surface layer of a spring wire from a surface to deep inside the spring wire. For example, shot peening is widely used for providing compressive residual stress on a surface layer of a spring wire in order to improve the fatigue resistance of a spring. By increasing the compressive residual stress at the surface layer by shot peening, breakage originating from the surface at an early time may be prevented. However, since the yield strength is increased according to increase in the hardness of a spring wire, the amount of plastic strain that can be provided on the surface layer by the shot peening is decreased, and a thick compressive residual stress layer becomes difficult to form. The thickness of the compressive residual stress layer is a distance from the surface to a position where the compressive residual stress is zero. On the other hand, according to the increase in design stress, combined stress of applied stress and residual stress (net stress applied to an inside of a spring wire) reaches a maximum at around a depth of 200 to 600 μm from the surface. This depth from the surface in a radial direction depends on the diameter of the spring wire, the degree of the applied stress, and the like. If inclusions with sizes of approximately 20 μm exist within this area, stress concentrates on the inclusions. The concentrated stress may exceed the fatigue strength of the spring wire and make the inclusions starting points of breakage. Accordingly, the following techniques were disclosed in order to solve these problems.
International Laid Open No. WO2005/081586 and Japanese Unexamined Patent Application Laid-open No. 2008-115468 disclose a method for induction heating a coiled member. However, these references are silent about characteristics such as material strength and metallic structure after induction heating, and effect of the induction heating is unknown.
A spring with superior fatigue resistance is disclosed in Japanese Unexamined Patent Application Laid-open No. 64-83644. This spring is produced by using an oil tempered wire rod in which an element such as V is added in the chemical composition of the steel that is specified in the JIS. The additional element increases toughness of the steel material by refining crystal grains and thereby improves the fatigue resistance; however, this increases the material cost.
A spring made of a silicon killed steel wire with superior fatigue characteristics is disclosed in Japanese Unexamined Patent Application Laid-open No. 2008-163423. The spring is obtained by coiling a steel material in which the amounts of Ba, Al, Si, Mg, and Ca are adjusted. In order to add these elements in balanced amounts, the process of steel refining is very difficult to control, whereby the production cost is high.
A spring with superior fatigue strength is disclosed in Japanese Unexamined Patent Application Laid-open No. 2005-120479. In this spring, the chemical composition of the steel is adjusted, and grain size is decreased while the size of inclusions is decreased because the inclusions may become starting points of fatigue failure. In this spring, the fatigue strength is increased, but the degree of the fatigue strength (maximum shear stress τ max=approximately 1200 MPa) is lower than a practical strength (τ max=approximately 1300 to 1400 MPa). The practical strength is required of lightweight and high strength valve springs of recent years. In addition, a method of further performing a nitriding treatment for obtaining higher fatigue strength is disclosed in Japanese Unexamined Patent Application Laid-open No. 2005-120479. The nitriding treatment can increase the surface hardness, whereby the fatigue resistance may be improved. In this method, iron nitrides are formed on a surface layer and must be completely removed after the nitriding treatment, because the iron nitrides may cause decrease in the fatigue strength. Therefore, the production process is complicated, and the cost of the nitriding treatment is high, whereby the production cost is high.
A spring steel wire with superior cold formability and high fatigue strength is disclosed in Japanese Unexamined Patent Application Laid-open No. 2-57637. This spring steel wire is obtained by adding Mo, V, and the like, to a chemical composition of a spring steel that is specified in the JIS and by austempering treatment. In this technique, the yield ratio (ratio of yield strength to tensile strength) is set to be not more than 0.85 in order to decrease tensile residual stress that may remain at the inner diameter side of a coil spring after the spring steel wire is cold formed. However, even when a spring wire with a yield ratio of not more than 0.85 is cold coiled and is then annealed, it is difficult to sufficiently decrease the tensile residual stress, which is generated after the cold coiling, from the surface to the inside. Therefore, even by subsequently performing shot peening, it is difficult to provide compressive residual stress from the surface to the deep inside, whereby improvement in fatigue resistance is limited.